As a kind of field-effect transistor, a thin film transistor in which a channel formation region is formed in a semiconductor layer fainted over a substrate having an insulating surface is known. Techniques for using amorphous silicon, microcrystalline silicon, and polycrystalline silicon for semiconductor layers in thin film transistors have been disclosed (see Patent Documents 1 to 5). A typical application of the thin film transistor is a liquid crystal television device in which the thin film transistor has been put to practical use as a switching transistor for a pixel that constitutes a display screen.
[REFERENCE]
[Patent Document]
    [Patent Document 1] Japanese Published Patent Application No. 2001-053283    [Patent Document 2] Japanese Published Patent Application No. H5-129608    [Patent Document 3] Japanese Published Patent Application No. 2005-049832    [Patent Document 4] Japanese Published Patent Application No. H7-131030    [Patent Document 5] Japanese Published Patent Application No. 2005-191546